With the increasing importance of environmental problems in recent years, the use of lithium-ion secondary batteries for use in electric vehicles (EV) and hybrid electric vehicles (HEV) in power storage systems that store surplus power realized by PV (Photovoltaic) generation is being investigated. In addition, lithium-ion secondary batteries are also receiving attention as an effective countermeasure against the type of environmental problems that arise from lead storage batteries.
Power storage systems of this type include a configuration in which a plurality of secondary battery packs made up by secondary batteries are arranged in parallel. However, when the plurality of secondary battery packs are arranged in parallel, the potential exists for a high current, that will flow to charge secondary batteries, to cause an accident. As a result, measures are taken to deal with current that is generated by voltage difference among the secondary battery packs, and the heat that is generated due to the flow of current in this secondary battery pack configuration, by either connecting each secondary battery pack to a load, or by disconnecting each secondary battery pack from the load (see Patent Document 1).
The power storage system described in Patent Document 1 is provided with a plurality of storage element rows arranged in parallel and performs an operation to connect the storage element row to a load on the condition that the voltage difference of the storage element row be within a predetermined range of values, whereby overcurrent or abnormal heat caused by voltage differences can be prevented.